Warner & Shine 2008

Warner & Shine 2008 - doi:10.1038/nature06519 L...

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LETTERS The adaptive significance of temperature-dependent sex determination in a reptile D. A. Warner 1 { & R. Shine 1 Understanding the mechanisms that determine an individual’s sex remains a primary challenge for evolutionary biology. Chromosome-based systems (genotypic sex determination) that generate roughly equal numbers of sons and daughters accord with theory 1 , but the adaptive significance of environmental sex determination (that is, when embryonic environmental condi- tions determine offspring sex, ESD) is a major unsolved prob- lem 2,3 . Theoretical models predict that selection should favour ESD over genotypic sex determination when the developmental environment differentially influences male versus female fitness (that is, the Charnov–Bull model) 4 , but empirical evidence for this hypothesis remains elusive in amniote vertebrates—the clade in which ESD is most prevalent 5 . Here we provide the first substantial empirical support for this model by showing that incubation tem- peratures influence reproductive success of males differently than that of females in a short-lived lizard ( Amphibolurus muricatus , Agamidae) with temperature-dependent sex determination. We incubated eggs at a variety of temperatures, and de-confounded sex and incubation temperature by using hormonal manipulations to embryos. We then raised lizards in field enclosures and quan- tified their lifetime reproductive success. Incubation temperature affected reproductive success differently in males versus females in exactly the way predicted by theory: the fitness of each sex was max- imized by the incubation temperature that produces that sex. Our results provide unequivocal empirical support for the Charnov– Bull model for the adaptive significance of temperature-dependent sex determination in amniote vertebrates. Why is an individual’s sex determined by environmental variables (environmental sex determination, ESD) in some species, but by chromosomal factors (genotypic sex determination; GSD) in others? GSD plausibly enhances parental fitness by generating equal invest- ment into sons versus daughters 1 , but the adaptive significance of ESD remains a major unresolved problem, particularly for amniote vertebrates 2,3 . The problem is not a lack of plausible hypotheses, but rather the difficulty of testing those ideas. Mathematical models pre- dict that ESD will be favoured by selection when an environmental variable (for example, temperature or photoperiod) differentially affects the fitness of sons versus daughters 4,6 . For example, the most common form of ESD is temperature-dependent sex determination (TSD), whereby incubation temperature determines offspring sex in many reptiles 5 and some fish 7 . The widely accepted Charnov–Bull model 4 predicts that TSD enhances parental fitness by matching offspring sex to incubation conditions; that is, eggs should produce sons when developing under conditions that promote high fitness for males, whereas eggs that encounter female-favourable conditions develop as daughters (see Fig. 1 for more detailed predictions).
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This note was uploaded on 08/24/2011 for the course ZOO 4926 taught by Professor Staff during the Spring '08 term at University of Florida.

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Warner & Shine 2008 - doi:10.1038/nature06519 L...

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